1. Field of the Invention
The present invention relates generally to an offset electrostatic printer which utilizes dehumidified air to extend the lifetimes of the print head and of the dielectric imaging member and to an offset electrostatic imaging process involving the utilization of dehumidified air.
2. Description of the Prior Art
In a typical electrostatic imaging process, a latent electrostatic image is formed on a dielectric charge retentive surface using a non-optical means, such as an electrostatic print head which generates ions by the corona discharge from a small diameter wire or point source. The dielectric surface can be either on the final image recording or receiving medium or on an intermediate transfer element, such as a cylindrical drum.
The latent electrostatic image is then developed by depositing a developer material containing oppositely charged toner particles. The toner particles are attracted to the oppositely charged latent electrostatic image on the dielectric surface. If the dielectric surface is on the final recording medium, then the developed image can be fixed by applying heat and/or pressure. If the dielectric surface is on an intermediate transfer element, however, then the developed image must first be transferred to the final recording medium, for example plain paper, and then fixed by the application of heat and/or pressure. Alternatively, the developed image may be fixed to the final recording medium by means of the high pressure applied between the dielectric-coated transfer element and a pressure roller, between which the final recording medium passes.
The intermediate transfer element in an offset electrostatic imaging process is typically a cylindrical drum made from an electrically conductive, non-magnetic material, such as aluminum or stainless steel, which is coated with a dielectric material. Suitable dielectric materials include polymers, such as polyesters, polyamides, and other insulating polymers, glass enamel, and aluminum oxide, particularly anodized aluminum oxide. Dielectric materials such as aluminum oxide are preferred to layers of polymers because they are much harder, and therefore, are not as readily abraded by the developer materials and the high pressure being applied. Metal oxide layers prepared by a plasma spraying or detonation gun deposition process have been particularly preferred as dielectric layers because they are harder and exhibit longer lifetimes than layers prepared using other processes.
One major problem encountered with currently available electrostatic printers of the ion deposition screen type has been the limited lifetime of the electrostatic aperture board. These types of electrostatic printers are disclosed in U.S. Pat. Nos. 3,689,935, 4,338,614 and 4,160,257. Such electrostatic printers have a row of apertures which selectively allow ionized air to be deposited onto a dielectric surface in an imagewise dot matrix pattern. It has been observed that a chemical debris tends to build up around the apertures and on the corona wire as a function of time and the humidity of the air. This chemical debris was found to be a crystalline form of ammonium nitrate. This particular chemical is created when air containing water molecules, such as is generally encountered, is ionized.
It has also been observed that, when an electrostatic printer of the type disclosed in U.S. Pat. No. 4,365,549 is operated in a moderately high relatively humidity, the surface conductivity of the dielectric drum increases where the ionized water molecules are deposited. The ionized water molecules are complexes containing hydronium ions. Water molecules in the air can become ionized by the corona wire in the ion deposition print head or by the A.C. scorotrons which are used to discharge residual charge on the drum. These conductive areas are observed on the final recording medium as weakly developed areas. This is believed to be caused by the more conductive surfaces leaking off their latent electrostatic images to the toner which has been made conductive during the development operation.
A number of methods have been suggested for alleviation of this problem of contaminant buildup. It has been suggested that the air being supplied to the corona discharge device first be filtered through a filter for ammonia in order to prevent the formation of ammonium nitrate. This method has not been found to be effective because it does not remove the water molecules in the air which under the influence of a corona discharge and in combination with other components of air form precursors to ammonium nitrate. Another method suggested for inhibiting formation of ammonium nitrate in an ion generator which includes a glow discharge device is to heat the glow discharge device above its intrinsic operating temperature at or near the ion generation sites.